Satellite mobile telephone cell departure prediction

ABSTRACT

A mobile station ( 4 ) in a satellite mobile telephone system predicts when it will move into another cell on the basis of one set of broadcast information thereby reducing the overall processing burden. The mobile station ( 4 ) can predict with a useful degree of certainty which broadcast control channel (BCCH) frequencies it should listen on when it wakes up on the basis of a stored map of the relative positions of cells. The broadcast information includes information about the current cell&#39;s ( 40 ) shape (V 1 , V 2 , V 3 , V 4 , V 5 , V 6 ) and translational (T) and rotational (R) motion.

FIELD OF THE INVENTION

The present invention relates to the predition of the time when a mobilestation will depart a cell of a satellite mobile telephone system.

BACKGROUND OF THE INVENTION

Known satellite mobile telephone systems include the Inmarsat-M system,the IRIDIUM™ system described in, for example EP-A-0365885, the ICO™system described in, for example, GB-A-2295296 and the ODYSSEY™ systemdescribed in, for example, EP-A-0510789. Whilst each of these systems iscellular in nature, they differ from terrestrial systems in that thecells move relative to the Earth's surface because each cell is definedby a beam from an orbiting satellite.

To extend battery life during the idle mode of a satellite mobiletelephone, the mobile telephone is desirably powered down for 95% to 98%of the time. Periodically, the mobile telephone wakes up briefly todetermine if broadcast control channels from satellites can be received.If so the mobile telephone checks for an incoming call. In idle mode,the mobile telephone needs to know when to hand over to another beamfrom the same or a different satellite, i.e. when to start listening toanother broadcast control channel.

If a mobile telephone does not know the time of the next handover or thenew broadcast control channel, it must frequently search a number offrequencies, for example 8. In particular, if on wake-up the broadcastcontrol channel is found to be weak or absent, the mobile telephone hasno way of knowing whether the signal is being blocked or interfered withor whether it is now being serviced by another beam.

It is desirable for a mobile telephone to check only two or threebroadcast control channel frequencies (one from a primary satellite andone or two from secondary satellites) instead of all possiblefrequencies. This minimises “on time” during each wake-up whilemaintaining the desired low duty cycle. Frequent wake-ups are desirableto minimise the time taken to detect incoming calls and the return ofsignals after an outage.

It is an aim of the present invention to overcome this problem.

SUMMARY OF THE INVENTION

Briefly stated, in a system according to the present invention, a mobilestation predicts when it will move into another cell on the basis of oneset of broadcast information thereby reducing the overall processingburden.

According to the present invention, there is provided a method ofoperation of a satellite mobile telephone system in which a plurality ofcells move across the surface of the Earth as a satellite orbits, themethod comprising the steps of: providing a mobile station withinformation related to the movement of a cell relative to the Earth'ssurface; and determining at the mobile station a prediction for the timewhen the mobile station will leave said cell on the basis of saidreceived information. The method may involve providing the informationto the mobile station by broadcasting the information from a satelliteto a cell, the information defining the geographical position of thecell, and receiving said information for the cell at the mobile station.

According to the present invention, there is also provided satellitemobile telephone system comprising transmitting means for transmittingcontrol data in a control channel, wherein the control data comprisesgeographical information defining the position of a cell associated withthe control channel.

According to the present invention, there is further provided a mobiletelephone for a satellite mobile telephone system, including memorymeans for storing control data including geographical informationrelated to the movement of a cell across the Earth's surface andprocessing means for processing said information to make a prediction ofwhen the mobile station will depart the current cell. The mobile stationmay include receiving means for receiving said geographical data,wherein said data relates to the cell in which the mobile station islocated.

Although the mobile station must perform some numerical calculations,these are not burdensome compared with repeated scanning of allbroadcast control channel frequencies.

If the cells differ in size or shape, the information preferablyincludes information regarding the extent of the cell. However, themobile station may include information regarding the size and shape ofeach cell and, in this case, the information need only include theidentity of the cell in which the mobile station is located. Thedesigner of a system according to the present invention will be expectedto make a design choice balancing the conflicting requirements oftransmission capacity and mobile station complexity.

If the shape of the cell is unchanging, or the mobile station storesinformation regarding the shapes of cells, the information will includeinformation regarding the traversing of the cell over the Earth'ssurface. In a system in which the cells rotate relative to the Earth'ssurface, the information will preferably include information regardingrotational movement of the cell.

If the mobile station is provided in advance with no geographicalinformation regarding the cell, the information preferably compriseslatitude and longitude co-ordinate values for the centre of the cell, avector defining the traversing of the cell over the surface of theEarth, co-ordinate values for the vertices of the cell relative to thecentre of the cell, at least one vector value defining rotation of thecell as it traverses the surface of the Earth and a time value.

Preferably, the information comprises a set of translational vectorsdefining the traversing of the cell over the surface of the Earth duringpredetermined sub-periods of a period during which said informationremains unchanged. The information may similarly comprise a set ofvectors defining the rotation of the cell. If the sub-periods arerelatively short, it may be sufficient for the mobile station merely toidentify the sub-period during which it will leave the cell. However, itis preferred that the mobile station derive a particular time, either byidentifying a point within one of the sub-periods or directly from thereceived information.

As a by product of the prediction of the cell departure time, the mobilestation will determine a prediction for the cell that it will beentering. Accordingly, the mobile station can identify the controlchannels that it should monitor after departing a cell.

Since a network will often be aware of a mobile station's location andthe movement of cells across the Earth, the mobile station preferablyonly need re-register with the network if its prediction is not correct,indicating that the mobile station has moved. Accordingly, there ispreferably a step of determining at the mobile station the correctnessof the prediction and performing a network registration process for themobile station in dependence on said determined correctness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the physical components of a satellitemobile telephone system;

FIG. 2 shows a mobile telephone partially cut away;

FIG. 3 is a schematic diagram of a satellite access node as shown inFIG. 1;

FIG. 4 is a data flow diagram for the system shown in FIG. 1;

FIG. 5 illustrates the movement of a cell over the Earth's surface;

FIG. 6 illustrates the movement of a cell in one five-minute period;

FIGS. 7( a) and (b) illustrate the position of a mobile station relativeto a cell at respectively the beginning and the end of a five-minuteperiod during which it leaves the cell;

FIG. 8 is a flow chart illustrating a method of determining the timewhen a mobile station will leave a cell;

FIG. 9 is a flow chart illustrating a preferred method of determiningthe time when a mobile station will leave a cell;

FIG. 10 is a graph showing errors resulting from approximations used inthe second method; and

FIG. 11 is a flow chart illustrating the operation of the mobile stationat the predicted cell departure time.

DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described, byway of example, with reference to the accompanying drawings.

Referring to FIG. 1, a satellite mobile telephone system comprises aplurality of satellite access nodes (SAN) 1 a, 1 b, 1 c interconnectedby a high capacity digital network 2 (hereinafter “the backbonenetwork”), a plurality of satellites 3 a, 3 b, a plurality of a mobiletelephones (UT) 4 a, 4 b, gateways 5 a, 5 b, 5 c providing connectionsbetween the SANs 1 a, 1 b, 1 c and other networks 6, a networkmanagement centre (NMC) 7, a satellite control centre (SCC) and atracking, telemetry and control station (TT&C) 9. The NMC 7, the SCC 8and the TT&C 9 are interconnected by a lower capacity digital network 10which is also connected to the backbone network 2. The other networks 6comprise the public switched telephone network (PSTN), cellulartelephone networks and the like.

The SCC 8 and the TT&C 9 control the operation of the satellites 3 a, 3b, for instance setting transmit power levels and transponder inputtuning, as directed by the NMC 7. Telemetry signals from the satellites3 a, 3 b are received by the TT&C 9 and processed by the SCC 8 to ensurethat the satellites 3 a, 3 b are functioning correctly.

During a telephone call, a UT 4 a, 4 b communicates with a satellite 3a, 3 b via a full duplex channel comprising a downlink channel and anuplink channel. The channels comprise TDMA time slots on a frequenciesallocated on initiation of the call. The satellites 3 a, 3 b are innon-geostationary orbits and comprise generally conventional satellites,such as the known Hughes HS601 model, and may include features asdisclosed in GB-A-2288913. Each satellite 3 a, 3 b is arranged togenerate an array of beams, one for each cell, covering a footprintbeneath the satellite, each beam including a number of differentfrequency channels and time slots.

Referring to FIG. 2, a UT 4 is generally similar to the units presentlyavailable for GSM networks and comprises a codec, a controller 16, amicrophone 10, a loudspeaker 11, a battery 12, a keypad 13, a radiofrequency interface, an antenna 14, a display 15 and subscriberidentification module (SIM) smart card.

The codec comprises a low bit-rate coder, which generates a speech bitstream at 3.6 kbits/s, together with a channel coder, which applieserror correction codes to the speech bit stream to produce an encodedbit stream at 4.8 kbits-s. The low bit-rate coder is a linear predictivecoder. The channel coder uses Viterbi coding. The codec also comprisescomplementary decoders.

The controller 16 comprises a microprocessor and RAM 16 a and ROMmemory. The microprocessor operates in accordance with a control programstored in the ROM memory. The controller can exchange control andtelemetry data with a SAN 1 using the radio frequency interface.

The SIM 16 includes a processor and a non-volatile memory which storesdata identifying the subscriber and data for use in encryptedcommunication.

Referring to FIG. 3, a SAN 1 comprises a dish antenna 20 mounted fortracking satellites, transmitter and receiver circuits 21 includingamplifiers, multiplexers, demultiplexers and codecs, a mobile satelliteswitching centre (MSSC) 22 including a controller 23, a visitor locationregister database (VLR) 24 and a voice mail box unit (VMB) 25. The MSSC22 is coupled for communications signals to the backbone network 2, to agateway and to the transmitter and receiver circuits 21. The controller23 is coupled for data signals to the VLR 24 and the VMB 25 and may alsosend and receive data signals via the backbone network 2.

The controller 23 responds to addresses on incoming communicationssignals, from the antenna 20, the gateway and the backbone network 2, bycontrolling the MSSC 22 to output the communications signals on theappropriate paths to their destinations, i.e. the antenna 20, thegateway or the backbone network 2.

The VLR 24 maintains a record of each of the subscribers registered withthe SAN 1. The VMB 25 provides storage space for voice mail messages forsubscribers.

Referring to FIG. 4, a database 30, called the home location register(HLR), contains records relating to each UT 4. The record contains theUT's identity (International Mobile Subscriber Identity or IMS), thecurrent status of the UT (whether it is “local” or “global” as will bedescribed in greater detail below), the geographical position of the UT,the home MSSC 22 a with which the UT is registered (to enable billingand other data to be collected at a single point), the currently activeSAN 1 a with which the UT is in communication via a satellite, anindividual encyphering key and the address of an associated VMB 25 alocation. If the UT 4 registers with the other SAN 1 b, its HLR recordis copied to the VLR 25 b at that SAN 1 b.

The HLR 30 may be located in the NMC 7 (see FIG. 1) or may bedistributed among the SANs 1 a, 1 b, 1 c (see FIG. 1).

Referring to FIGS. 1 to 4, a UT 4 may be registered with one of twodistinct statuses; “local” in which the UT 4 is permitted to communicateonly through one local area or part of the satellite system network, and“global”, which entitles the UT 4 a to communicate through any part ofthe satellite mobile telephone system.

The UT 4 performs an automatic registration process, of the kind wellknown in the art of cellular terrestrial communications, on eachoccasion when the UT 4 is used for an outgoing call, when the UT 4 isswitched on and periodically whilst the UT 4 a is operating. As isconventional, the registration process takes the form of transmitting ofa signal identifying the UT 4 (e.g. by transmitting its telephone numberon a common hailing or signalling channel).

The transmitted signal is picked up by one or more of the satellites 3a, 3 b. Under normal circumstances, the signal is picked up by aplurality of satellites 3 a, 3 b, and the received signal strength ortime of arrival are transmitted, together with the identity of the UT 4and the identity of the satellite 3 a, 3 b receiving the signal, to theHLR 30 via the MSSCs 22 a, 22 b of the SANs 1 a, 1 b for which thesatellites 3 a, 3 b are in communication.

The HLR 30 calculates, on the basis of the received-signal arrival timeand detected doppler effects, the terrestrial position of the UT 4 whichis then stored in the UT's record. The identity of the SAN 1 a, 1 b, 1 cmost suitable for communicating with the UT 4 a is also stored. This istypically found by comparing the stored position of the UT 3 a with thepositions of each of the SANs 1 a, 1 b, 1 c and selecting the nearest.However, account may also or instead be taken of the strengths of thesignals received via the satellites 3 a, 3 b, 3 c, or of other factorssuch as network congestion which may result, in borderline cases, in theselection of a SAN 1 a, 1 b, 1 c which is not geographically closest tothe UT 4 a. The identity of the allocated SAN 1 a is then stored in theUTs record in the HLR 30. Once the HLR record has been updated, it iscopied down to the VLR 24 a of the selected SAN 1 a.

Voice mail for the UT 4 is routed to the VMB 25 a at the SAN 1 a whichis the UT's home SAN via the MSSC 22 a. The voice mail can be retrievedby he UT4 via the MSSC 22 a from the VMB 25 a.

Referring to FIG. 5, as a satellite 3 orbits the Earth, a cell 40traverses the planet's surface from a first location at time t₁ to asecond location at time t₂. Consequently, a stationary UT 4 will bewithin the cell 40 for only a limited period of time. As the cell 40traverses, it also rotates.

The UT 4 always knows its own position. This information is transmittedto the UT 4 by the current SAN 1. The determination of the UT's positionmay be performed, as mentioned above, on the basis of signal delaythrough one satellite 3 and the detected doppler shift. Alternatives areto determine the position of the UT on the basis of received signaldelays via two or more satellites and to determine the position of theUT on the basis of doppler shifts in signal receive via a plurality ofsatellites. The UT 4 could be combined with a receiver for a navigationsystem such as GPS. In this case, the network does not need to transmitto the UT 4 its position.

The SAN 1 broadcasts via the satellite 3 to the cell in a broadcastcontrol channel (BCCH) the latitude and longitude co-ordinates Φc, Lc ofthe centre of the cell 40. These broadcasts take place once or twice inevery minute so that a UT 4 can rapidly acquire the co-ordinateswhenever it becomes active. Φc and Lc are updated every 40 minutes andare correct at a time t₀, 20 minutes ahead of the time when they werefirst broadcast. One set of latitude and longitude co-ordinates Φc, Lcis broadcast for 40 minutes, i.e. from t₀−20 to t₀+20. In practice, theUT 4 usually uses one set of co-ordinates Φc, Lc for 5 to 25 minutesbecause the UT 4 will generally have transferred to another cell withinthese time limits. However, occasionally, one set co-ordinates Φc, Lcwill be used for a longer period, up to 35 minutes.

With the satellite in a medium height orbit, a 40-minute interval spansthe maximum time that a stationary UT 4 will be in one cell 40.

In addition to Φc, Lc, the satellite 3 broadcasts a set T of velocityvectors defining the translational motion of the cell centre during thepresent 40-minute interval, the co-ordinates of the vertices 41, . . . ,46 of the cell 40. The velocity vectors comprise X, i.e. east-west, andY, i.e. north-south, components.

As the cell 40 moves over the Earth's surface, it rotates. The rotationof the cell 40 is defined by a set of velocity vectors R for three V₁,V₂, V₃ of the six vertices of the cell 40. Information regarding onlythree vertices is required because the cell 40 is always symmetrical andthe vectors for the other three vertices V₄, V₅, V₆ are the vectors forvertices V₁, V₂, V₃ respectively but rotated through 180°.

The information required by the UT 4 is transmitted in the followingformat:

Beam centre latitude Φc to 0.044° (5 km)  12 bits Beam centre longitudeLc to 0.044° (5 km)  13 bits T 110 bits 6 vertices 144 bits R  90 bitsEpoch time (t₀) HH:MM:SS (BCD)  13 bits Total 382 bits

A method of determining when a UT 4 will now be described with referenceto FIGS. 6, 7 and 8 and Tables 1 and 2.

TABLE 1 T vector values broadcast and used in calculations. Time t atthe end of (Exact) (Exact) period, Broadcast Broadcast Y minutes Xvelocity X velocity used velocity Y velocity used Period N from t₀values in calculations values in calculations 1 −15  X(−15)  X(−15) Y(−15)  Y(−15) 2 −10 (X(−15) + X(−5))/2 (Y(−15) + Y(−5))/2 3  −5 X(−5)X(−5) Y(−5) Y(−5) 4  0 X(0)  X(0)  Y(0)  Y(0)  5  5 X(5)  X(5)  Y(5) Y(5)  6  10 (X(15) + X(5))/2 (Y(15) + Y(5))/2 7  15 X(15) X(15) Y(15)Y(15) 8  20 X(15) Y(15)

TABLE 2 R vector values broadcast and used in calculations. Time t atend of (Exact) period, (Exact) X velocity Broadcast Y velocity Periodminutes Broadcast used in Y velocity used in N from t₀ X velocitycalculations values calculations 1 −15  X(−15)  X(−15) Y(−15)  Y(−15) 2−10   X(−15)  Y(−15) 3 −5 X(0)  Y(0)  4  0 X(0)  X(0)  Y(0)  Y(0)  5  5X(0)  Y(0)  6 10 X(15) Y(15) 7 15 X(15)  X(15) Y(15)  Y(15) 8 20 X(15)Y(15)

The velocity values are in km/min.

When the UT 4 first becomes active in a cell 40, its controller performsthe following method:

-   1. At step s1, the UT 4 determines the present time within the    current 40-minute interval.-   2. Once the present time has been determined, the UT 4 calculates    the latitude and longitude of the cell centre C at the end of the    current five-minute period, step s2 and at the end of each period    until the UT 4 leaves the cell 40. The latitude of the cell centre    can be determined at the end of the nth period using the equation:

$\begin{matrix}{\Phi_{Cn} = {\Phi_{C4} + {\sum\limits_{N = 0}^{n}{X_{N} \cdot 0.045}}}} & (1)\end{matrix}$

-    where X_(N) is the approximated X velocity value from Table 1 for    the cell for the Nth five-minute period, Φ_(C4) is the latitude of    the cell 40 when t=0, i.e. at the end of the fourth period, and    Φ_(Cn) is the latitude value at the end of the nth five-minute    period.    -   The longitude of the cell can be similarly calculated for each        five-minute period from the current time until it is determined        that the UT 4 would be outside the cell 40 using the equation:

$\begin{matrix}{L_{Cn} = {L_{C4} + {\sum\limits_{N = 0}^{n}{Y_{N} \cdot 0.045}}}} & (2)\end{matrix}$

-   -   where Y_(N) is the approximated Y velocity value from Table 1        for the cell for the Nth five-minute period, L_(C4) is the        longitude when t=0, and L_(Cn) is the longitude value at the end        of the nth five-minute period.    -   In practice the Φ_(Cn) and L_(Cn) values are accumulated and in        all but the initial calculation, so that if further calculations        are required in the present interval the following equations are        used instead of Equations (1) and (2):        Φ_(Cn)=Φ_(C(n−1)) +X _(n)0.045        and        L _(Cn) =L _(C(n−1)) +Y _(n)0.045    -   Since, the position of the cell centre C at the middle of the        40-minute interval is transmitted in the BCCH, the cell velocity        vectors for periods 1 to 4 must be rotated by 180° either before        or after being broadcast.

-   3. After each calculation of cell centre C latitude and longitude,    it is necessary to determine whether the UT 4 is outside the cell    40. This is carried out in the following manner:    -   a) First, at step s3, the Cartesian co-ordinates (x_(UT),        y_(UT)) of the UT 4 relative to the cell centre are calculated.        Although a cell exists on a spherical surface, usable results        can be obtained by treating the cell as being flat. The UT 4        calculates its Cartesian co-ordinates using the following        approximate equations:        x _(UT)=111.32·(L _(UT) −L _(Cn))·cos Φ_(UT)  (3)        and        y _(UT)=111.32·[(Φ_(UT)−Φ_(C) _(n) )+7.83·10⁻⁵ ·x _(UT)(L _(UT)        −L _(Cn))sin Φ_(Cn)]  (4)    -    where L_(UT) and Φ_(UT) are the longitude and latitude of the        UT 4. The UT 4 is informed of its longitude and latitude each        time it performs an update with the network. (111.32 is the        great circle distance on the Earth's surface in km corresponding        to an angular separation of 1°).        -   Above about 65° latitude, the errors arising from the use of            Equations (3) and (4) increases. Suitable approximate            expressions for use beyond 65° latitude north or south are:            x _(UT)=111.32·(90−Φ_(UT))·sin(L _(UT) −L _(Cn))            and            y _(UT)=111.32·[Φ_(UT)·cos(L _(UT) −L            _(Cn))−Φ_(Cn)+90(1−cos(L _(UT) −L _(Cn)))]    -   b) Since the cell rotates as it traverses, it is necessary to        determine the co-ordinates of the vertices V₁ . . . V₆ of the        cell 40. This is performed in step s4. The R values in Table 2        are used for these calculations which employ initially the        equations:

$\begin{matrix}{x_{V_{j_{N}}} = {x_{V_{j_{4}}} + {\sum\limits_{N = 0}^{n}{{Xj}_{N} \cdot 0.045}}}} & (5) \\{and} & \; \\{y_{V_{j_{N}}} = {y_{V_{j_{4}}} + {\sum\limits_{N = 0}^{n}{{Yj}_{N} \cdot 0.045}}}} & (6)\end{matrix}$

-   -   -   where x_(VjN) and y_(VjN) are the co-ordinates of the jth            vertex at the end of the Nth five-minute period and X_(jN)            and Y_(jN) are the R vector values from Table 2 for the jth            vertex.        -   However, as in the case of the cell centre position, the            x_(VjN) and y_(VjN) are accumulated and similar simplified            equations are used for subsequent calculations.

    -   c) At step s5, the vectors V_(1N) . . . V_(6N) mapping the UT's        position onto each of the vertices are derived, thus:        v _(jN)=(x _(V) _(jN) −x _(UT)) x +(y _(V) _(jN) −y _(UT)) y          (7)        -   where x and y are orthogonal unit vectors.

    -   d) At step s6, the angles θ_(jN), in a consistent direction,        between adjacent vectors v₁ . . . v₆ are calculated and, if the        calculated angle is greater than 180°, the UT 4 must be outside        the cell 40. The angle between adjacent vectors is determined        using the following equation:

$\begin{matrix}{\theta_{jN} = {{\tan^{- 1}\left( \frac{x_{{Vj}_{N}}}{y_{{Vj}_{N}}} \right)} - {\tan^{- 1}\left( \frac{x_{{V{({j - 1})}}_{N}}}{y_{{V{({j - 1})}}_{N}}} \right)}}} & (8)\end{matrix}$

-   -   -   If j=1 then j−1 is replaced by 6. There is a real            possibility of a divide by zero error occurring here so            special care must be taken to check the values of the            divisors before performing the divisions.        -   The appropriate angle obtained from tan⁻¹(x/y) can be            selected on the basis of the signs of x and y. If θ_(jN) is            negative, then the equivalent positive angle should be used,            i.e. 360°+θ_(jN).        -   An arctan lookup table may be stored in the UT controller's            ROM to avoid the need to calculate these values. By            including an indication of the signs of the x and y values            in the lookup table addressing scheme, the lookup table can            further reduce the burden on the UT controller's            microprocessor.        -   If an angle θ_(jN) is determined to be greater than or equal            to 180° (step s7), the process moves directly to step s8. If            this is not the case, it is determined whether all the            angles have been tested at step s9. If all the angles have            been tested, the process returns to step s2 otherwise the            process returns to step s6 and next angle is tested.        -   In practice, the motion of the cell 40 may mean that the UT            4 will leave the cell 40 via one of a subset of edges,            assuming the UT 4 is stationary. If this is the case, the            number of vectors v_(n) can be reduced. However, the            presently described algorithm has the advantage of            universality.

-   4. Once the five-minute period, during which the UT 4 leaves the    cell has been identified, a more closely approximate time for the    UT's departure can be calculated.    -   The first stage of this process is to determined the point on        the cell's boundary which passes through the location of the UT        4 (step s8). The edge linking the vectors v_(n), v_(n−1),        separated by 180° or more, is the edge that passes through the        UT's position. If the angle θ_(jN) was determined to be 180°, no        further calculations are necessary because this condition means        that the UT 4 will leave the cell 40 at the boundary between two        five-minute periods.    -   The rotation of the cell can be ignored for this calculation.        The x co-ordinate of the crossing point x_(X) is obtained from:

$\begin{matrix}{x_{X} = {\left( {y_{UT} - {\frac{X_{N}}{Y_{N}} \cdot x_{UT}}} \right) - \left( {y_{{Vj}_{N}} - \frac{\left( {y_{{Vj}_{N}} - y_{{V{({j - 1})}}_{N}}} \right) \cdot x_{{Vj}_{N}}}{\left( {x_{{Vj}_{N}} - x_{{V{({j - 1})}}_{N}}} \right)}} \right)}} & (9)\end{matrix}$

-   -   The y co-ordinate y_(X) is then be obtained from:

$\begin{matrix}{y_{X} = {{\frac{X_{N}}{Y_{N}} \cdot x_{X}} + \left( {y_{UT} - {\frac{X_{N}}{Y_{N}} \cdot x_{UT}}} \right)}} & (10)\end{matrix}$

-   -   where X_(N) and Y_(N) are the cell velocity vector components        (Table 1) for the five-minute period during which the UT 4 will        leave the cell 40.    -   The co-ordinates of the UT 4 and the crossing point are then        used at step s10 to calculate the time for the crossing from:

$\begin{matrix}{t_{X} = {t_{N} - \frac{\left( \sqrt{\left( {x_{UT} - x_{X}} \right)^{2} + \left( {y_{UT} - y_{x}} \right)^{2}} \right)}{\sqrt{X_{N}^{2} + Y_{N}^{2}}}}} & (11)\end{matrix}$

-   -   where t_(N) is the time at the end of the five-minute period        during which the UT 4 will leave the cell 40.

A preferred method of determining when a UT 4 will leave a cell will nowbe described with reference to Tables 3 and 4.

TABLE 3 T vector values broadcast and used in calculations. Time t atthe centre of X period, (Exact) velocity (Exact) Y velocity Periodminutes Broadcast used in Broadcast Y used in N from t₀ X velocitycalculations velocity calculations 1 −20 X(−10) Y(−10) 2 −15$\frac{\left( {{X\left( {- 10} \right)} + {X\left( {- 5} \right)}} \right)}{2}$$\frac{\left( {{Y\left( {- 10} \right)} + {Y\left( {- 5} \right)}} \right)}{2}$3 −10 X(−10) X(−5) Y(−10) Y(−5) 4 −5 X(−5) (X(−5) + X(0))/2 Y(−5)(Y(−5) + Y(0))/2 5 0 X(0) X(0) Y(0) Y(0) 6 5 X(5) (X(0) + X(5))/2 Y(5)(Y(0) + Y(5))/2 7 10 X(10) X(5) Y(10) Y(5) 8 15 (X(5) + X(10))/2 (Y(5) +Y(10))/2 9 20 X(10) Y(10)

TABLE 4 R vector values broadcast and used in calculations. Time t atcentre of period, (Exact) X velocity (Exact) Y velocity Period minutesBroadcast used in Broadcast used in N from t₀ X velocity calculations Yvelocity calculations 1 −20  X(−15)  Y(−15) 2 −15 X(−15)  X(−15) Y(−15) Y(−15) 3 −10  X(−15)  Y(−15) 4 −5 X(0)  Y(0)  5 0 X(0)  X(0)  Y(0) Y(0)  6 5 X(0)  Y(0)  7 10 X(15) Y(15) 8 15 X(15)  X(15) Y(15)  Y(15) 920 X(15) Y(15)

The velocity values are in km/min.

In this embodiment, the control program stored in the UT controller'sROM is different.

Referring to FIG. 9, when a UT 4 receives the broadcast information, theinformation is processed by its controller in the following manner:

-   1. At step s21, the UT 4 determines the present time within the    current 40-minute interval.-   2. At step s22, the Cartesian co-ordinates (x_(UT0), y_(UT0)) of the    UT 4 relative to the cell centre at t=0 are calculated. Although a    cell exists on a spherical surface, usable results can be obtained    by treating the cell as moving over a flat surface. The UT 4    calculates its Cartesian co-ordinates using the following    approximate equations:    x _(UT0)=111.32·(L _(UT) −L _(C))·cos Φ_(UT)  (12)    and    y _(UT0)=111.32·[(Φ_(UT)−Φ_(C))+7.83·10⁻⁵ ·x _(UT0)(L _(UT) −L    _(C))sin Φ_(C)]  (13)-    where L_(UT) and Φ_(UT) are the longitude and latitude of the UT 4.    The UT 4 is informed of its longitude and latitude each time it    performs an update with the network. (111.32 is the great circle    distance on the Earth's surface in km corresponding to an angular    separation of 1°).    -   Above about 65° latitude, the errors arising from the use of        Equations (12) and (13) increases. Suitable approximate        expressions for use beyond 65° latitude north and south are:        x _(UT0)=111.32·(90−Φ_(UT))·sin(L _(UT) −L _(C))        and        y _(UT0)=111.32·[Φ_(UT)·cos(L _(UT) −L _(C))−Φ_(C)+90(1−cos(L        _(UT) −L _(C)))]-   3. At step s22, the present time t_(p) with respect to t₀ is    calculated as the difference between the UT's clock time and the    broadcast value of the epoch time t₀.-   4. At step s23, the Cartesian co-ordinates (x_(UT), y_(UT)) of the    UT 4 relative to the cell centre at t=t_(p) are calculated using:    x _(UT) =x _(UT0) −X _(TN) ·t _(p)  (14)    and    y _(UT) =y _(UT0) −Y _(TN) ·t _(p)  (15)-   5. where X_(TN) and Y_(TN) are values selected from Table 3 based in    t_(p). Though in fact it is the cell vertices and cell centre that    translate over the Earth while the UT 4 remains stationary (hence    the minus signs in Equations (14) and (15)), the computations in    step s23 and after are simpler and the maximum coordinate values    smaller if the cell provides the reference frame for the    coordinates.-   6. At step s24, the coordinates of the vertices of the cell 40 with    respect to the centre of the cell 40 are corrected for cell rotation    as follows:    x _(Vnt) =x _(Vn0) +X _(RN) ·t _(p)  (16)    and    y _(Vnt) =y _(Vn0) +Y _(RN) ·t _(p)  (17)    -   where t_(p) is the present time with respect to t₀, x_(Vnt) and        y_(Vnt) are the x and y co-ordinates of the nth vertex at time        t=t_(p), x_(Vn0) and y_(Vn0) are the x and y co-ordinates of the        nth vertex at t=t₀ (i.e. as broadcast), and X_(RN) and Y_(RN)        are values selected from Table 4 on the basis of t_(p).-   7. The cell 40 motion always has a positive X (i.e. eastwards)    component.

Accordingly, it can be inferred that the UT 4 cannot be crossed by acell edge whose ends are both east of the UT 4. Similarly, edges of thecell 40 to the north and the south of the UT 4 can be ignored when thecell 40 is moving northwards (+Y) or southwards (−Y) respectively.Therefore, at step s25, the edges of the cell 40, which cannot pass theUT 4, are discarded for the purposes of further calculations.

-   8. The handover point x_(X), y_(X) is on the intersection between    two lines: one connecting two vertices, V_(a) and V_(b), and another    connecting the present UT position x_(UT),y_(UT) with a future UT    position x_(f), y_(f) selected to be outside the cell. At step s26,    selecting the time to x_(f), y_(f) (with distances calculated from    the X and Y values applicable halfway there) can be done in any of    at least three ways: use a worst-normal-case value of 25 minutes;    use a value specific to the maximum diameter of each of the cell    types, from a lookup table (the cell type broadcast on BCCH); or use    a trial value and iterate if necessary.-   9. The coordinates x_(X) and y_(X) are calculated using:

$\begin{matrix}{x_{X} = \frac{{\left( \frac{y_{f} - y_{UT}}{x_{f} - x_{UT}} \right) \cdot x_{UT}} - {\left( \frac{y_{Vb} - y_{Va}}{x_{Vb} - x_{Va}} \right) \cdot x_{Va}} + y_{Va} - y_{UT}}{\left( \frac{y_{f} - y_{UT}}{x_{f} - x_{UT}} \right) - \left( \frac{y_{Vb} - y_{Va}}{x_{Vb} - x_{Va}} \right)}} & (18) \\{and} & \; \\{y_{X} = {{\left( \frac{y_{f} - y_{UT}}{x_{f} - x_{UT}} \right)\left( {x_{X} - x_{UT}} \right)} + y_{UT}}} & (19)\end{matrix}$

-   -   For the calculated handover point to be valid, the following        conditions must be met:        x _(f) ≦x _(X) ≦x _(UT)        x _(Va) ≦x _(X) ≦x _(Vb)        y _(f) ≦y _(X) ≦y _(UT)        y _(Va) ≦y _(X) ≦y _(Vb)    -   If these conditions are not met, another vertex pair or a new        future point x_(f), y_(f) must be chosen.

-   10. At step s28, the distance from the present UT position to the    future boundary-crossing point is calculated using:    R _(X)=√{square root over ((x _(UT) −x _(X))²+(y _(UT) −y    _(X))²)}{square root over ((x _(UT) −x _(X))²+(y _(UT) −y    _(X))²)}  (20)

-   11. At step s29, an initial estimate t_(x)′ of the time at    boundary-crossing with respect to t₀ is calculated using:

$\begin{matrix}{t_{x}^{\prime} = \frac{R_{X}}{\sqrt{X_{p}^{2} + Y_{p}^{2}}}} & (21)\end{matrix}$

-    in which X_(p) and Y_(p) are the exact broadcast velocities for the    period in which t_(p) falls, selected from Table 3 according to the    value of t_(p).    -   If |t_(p)|>12.5 minutes, Table 3 shows that no exact X_(p) and        Y_(p) are broadcast. In such cases, the X and Y values for + and        −10 minutes can be used to calculate t_(x)′. In the presently        described system, these cases occur only in the extended fringe        of the coverage are (i.e. below 10° elevation in beams at the        edge of the area covered by one satellite), the resulting loss        of accuracy is acceptable. In all but edge beams, the coordinate        update period could be reduced from the nominal maximum of 40        minutes to 20 or 25 minutes so that in all cases |t_(p)|<12.5        minutes. Alternatively, the X_(p) and Y_(p) values for        |t_(p)|<12.5 minutes could be broadcast in edge beams.-   12. If t_(X)′ is in the same five-minute period as t_(p), then X_(p)    and Y_(p) are sufficiently accurate and, therefore, t_(X)′ is also    sufficiently accurate. At step s30, a test for this condition is    made; if it exist steps s31 and s32 are bypassed and, at step s33, a    timer is set to wake up the UT 4 one minute before the predicted    crossing time (if t_(X)′−t_(p) is not already less than one minute).    This procedure give an immediate result when a crossing is imminent.-   13. If t_(X)′ is not in the same five-minute period as t_(p), then    at steps s31, a rate pair X_(H) and Y_(H) are selected from Table 3    at a time halfway between t_(p) and t_(X)′ to the nearest five    minutes. This selection process is illustrated by Table 5 below.    -   At step s32, the rate pair thus selected is then used to        calculate a more accurate value of crossing tim t_(X) using:

$\begin{matrix}{t_{x}^{\prime} = \frac{R_{X}}{\sqrt{X_{p}^{2} + Y_{p}^{2}}}} & (22)\end{matrix}$

-   -   Although the initial crossing-time estimate t_(X)′ may not be as        accurate as is required, its accuracy is sufficient to select        X_(H) and Y_(H) for calculating t_(X) with a useful degree of        accuracy. Once t_(X) has been calculated, the program flow moves        on to step s33.

TABLE 5 Time t at the centre of t_(X)', initial period, X velocity X_(H)Y velocity Y_(H) estimated Period minutes used to calculate used tocalculate crossing time N from t_(p) accurate t_(X) accurate t_(X) −7.5to −2.5 4 −5 X(−5) Y(−5) −2.5 to 2.5  5 0 (X(−5) + X(0))/2 (Y(−5) +Y(0))/2 2.5 to 7.5 6 5 X(0) Y(0)  7.5 to 12.5 7 10 (X(0) + X(5))/2(Y(0) + Y(5))/2 12.7 to 17.5 8 15 X(5) Y(5) 17.5 to 22.5 9 20 (X(5) +X(10))/2 (Y(5) + Y(10))/2

It is clear that a number of approximations have been used in thecalculation of t_(x). However, the errors induced by the use of theseapproximations do not produce a significantly adverse effect as shown byFIG. 10. The small size of the errors shown in FIG. 10 demonstrate thatthe technique of using only one rate value accurate at a time halfwaybetween that of a known position and that when a new position estimateis wanted gives small position errors with quite simple equations, suchas Equations (14) and (15)

Having calculated a predicted time for leaving a cell, by whatevermethod, the UT 4 operates as follows.

Referring to FIG. 11, when the UT 4 is woken up by the timer, it firstidentifies the cell it should now be entering using its knowledge of thecell edge through which it predicted that it would pass and an internalmap of relative cell positions (step s34).

Once the UT 4 has woken up, it scans the BCCH frequencies for the cellit is leaving and the cell it expects to be entering (step s35). Ifinitially, the BCCH for the current cell is not received, the UT 4determines that it has moved and starts scanning all BCCH frequencies todetermine which it should now use and then performs a registrationroutine, as hereinbefore described, with the network (step s36).

If the BCCH for the current cell 40 is found, the UT 4 monitors the BCCHfrequency for the current cell and the BCCH frequency for the cell itexpects to be entering, until the BCCH for the new cell is detected orthe BCCH for the current cell is lost (steps s37 and s38). If the BCCHfor the current cell is lost without the BCCH for the new cellappearing, the UT 4 determines that it has moved and scans all BCCHfrequencies to determine which it should now use and then performs aregistration routine with the network (step s36). At step s39, thesignal strengths of the BCCH's are compared until they are approximatelyequal. If the time when the two BCCH's have the same strength differssignificantly from the predicted departure time (step s40), the UT 4determines that it has moved and re-registers (step s41) so that thenetwork becomes aware of its new location. The approximations andassumptions used in the cell departure predictions will generally resultin some difference between the predicted and actual departure times.However, if the difference is greater than some amount, determined bythe details of the particular system, it can be safely assumed that theUT 4 has moved.

If the BCCH for the expected new cell is received as expected, the UT 4does not perform a registration routine with the network.

It is not necessary for the UT 4 to re-register on entering a cell asexpected because the network knows both the position of the UT 4 and theposition at any given time of all of the beams. Consequently, thecorrect beam can be selected for signalling to the UT 4 even if it hasnot re-registered for some time.

Since, a stationary UT does not need to re-register on entering a newcell, there is a reduction in the demand for system resources foradministrative purposes.

When the UT 4 enters a new cell, it receives the cell position andmovement data for the new cell, repeats the calculations set out aboveand re-enters its dormant state.

It will be appreciated that many modifications may be made to theabove-described embodiments. For example, the period and intervaldurations may be different. It may be found that it is sufficient todetermine the period during which the UT leaves a cell and wake up theUT at the start of that period.

Additional information could be transmitted to the UT. For example, inorder to reduce the burden on the UT's controller, the values for cosΦ_(UT) could be sent to the UT with its location. Also, 7.83×10⁻⁵ sinΦ_(C) could be broadcast in the BCCH.

The skilled person will also appreciate that the present invention maybe embodied using other co-ordinate systems, e.g. spherical polarco-ordinates, and that the cell translation and rotation may bedescribed as linear functions of time, in which case the UT need onlyknow the constants of the linear functions for the current cell todetermine the current cell's position at any time.

1. A method of operating a satellite mobile telephone in which aplurality of cells move across the surface of the Earth as a satelliteorbits, comprising: subsequent to completion of a registration process,receiving data related to a position and movement of a cell relative tothe Earth's surface; determining a position of mobile telephone; andwhile the mobile telephone is not actively processing a telephone call,using the received data and the position of the mobile telephone todetermine, at the mobile telephone, a prediction for the time when themobile telephone will leave the cell.
 2. The method of claim 1, furthercomprising initiating a cell selection operation at the predicted timewhen the mobile telephone will leave the cell.
 3. The method of claim 1,wherein the information is provided to the mobile telephone bybroadcasting the information defining a geographical position of aselected cell from a satellite to the selected cell, and receiving thedata for the selected cell at the mobile.
 4. The method of claim 1,wherein the data includes information regarding the extent of the cell.5. The method of claim 1, wherein the data comprises a set oftranslational vectors defining the traversing of the cell over thesurface of the Earth during a plurality of predetermined sub-periods ofa period during which the information remains unchanged.
 6. The methodof 5 wherein determining a prediction comprises the mobile telephonepredicting a selected one of the plurality of sub-periods during whichit will leave the cell on the basis of the members of the set oftranslational vectors.
 7. The method of claim 6 wherein the mobiletelephone predicts the time within the selected sub-period when it willleave the cell.
 8. The method of claim 1, further comprising selectingat the mobile telephone a control channel to be monitored.
 9. The methodof claim 1, further comprising determining at the mobile telephone thecorrectness of the prediction and performing a network registrationprocess for the mobile telephone in dependence on the determinedcorrectness.
 10. The method of claim 1, wherein the received datarelated to the movement of the cell relative to the Earth's surface isreceived from the satellite.
 11. The method of claim 10 wherein thereceived data is received from the satellite via a control channel. 12.The method of claim 1, wherein the data related to the movement of thecell comprises data related to a position of a central portion of thecell and a translational vector indicating a movement of the centralportion of the cell.
 13. The method of claim 1, wherein the data relatedto the movement of the cell is calculated for a predetermined period oftime.
 14. The method of claim 13 wherein the data related to themovement of the cell is calculated for a plurality of predeterminedperiods of time covering a time range that exceeds the prediction forthe time when the mobile telephone will leave the cell.
 15. The methodof claim 1, wherein mobile telephone is communicating with a selectedsatellite access node (SAN) via the satellite, the method furthercomprising receiving location data indicating a location of the mobiletelephone from the SAN via the satellite.
 16. The method of claim 15wherein the mobile telephone uses the location data in combination withthe received data related to the movement of the cell to determine atthe mobile telephone the prediction for the time when the mobiletelephone will leave the cell.
 17. The method of claim 1, wherein thesatellite mobile telephone has an awake mode and a power-down modewherein receiving data subsequent to completion of the registrationprocess is performed during a period in which the satellite mobiletelephone changes from the power-down mode to the awake mode.
 18. Themethod of claim 1, wherein the satellite mobile telephone has an awakemode and a power-down mode, the method further comprising during aperiod in which the satellite mobile telephone changes from thepower-down mode to the awake mode, scanning frequency channelsassociated with a satellite with which the satellite mobile telephonewas previously communicating and frequency channels associated with asatellite with which the satellite mobile telephone is expected tocommunicate upon leaving the cell.
 19. The method of claim 18, furthercomprising selecting the satellite with which the satellite mobiletelephone was previously communicating or the satellite with which thesatellite mobile telephone is expected to communicate based on thesatellite with greatest signal strength.
 20. The method of claim 18,further comprising selecting the satellite with which the satellitemobile telephone was previously communicating or the satellite withwhich the satellite mobile telephone is expected to communicate based onthe prediction for the time when the mobile telephone will leave thecell if the satellite mobile telephone was previously communicating andthe satellite with which the satellite mobile telephone is expected tocommunicate have approximately equal signal strength.
 21. The method ofclaim 18, further comprising scanning all frequency channels if nocommunication is established with the satellite with which the satellitemobile telephone was previously communicating or the satellite withwhich the satellite mobile telephone is expected to communicate uponleaving the cell.